Pen



FIG. 1 is a perspective view of the design;

FIG. 2 is an elevational view of the front of the design, the rear and both sides being identical to the front;

FIG. 3 is an enlarged plan view of the bottom of the design; and,

FIG. 4 is an enlarged plan view of the top of the design. 

The ornamental design for a pen, as shown and described. 